1. Field of the Invention
This invention relates generally to the field of communications and signal processing. In particular, the invention relates to a technique for eliminating artifacts in an output signal produced, for example, by systems employing multipath reception techniques.
2. Related Art
Multipath reception relates to obtaining and processing radio signals from one of multiple transmission paths or channels. Generally, the term multipath reception is applied to systems that incorporate multiple antennas (for spatial or antenna diversity) and systems that receive signals on multiple frequency channels (for frequency diversity).
In motor vehicles the radio system may employ antenna diversity in the form of multiple spatially separated antennas. For example, one antenna may be built into a window, while another antenna may rise from an exterior surface of the motor vehicle. When the antenna diversity radio system operates, a receiver system selects one of the antennas as the source of radio signals for processing based on predefined criteria. For example, the criteria may be received signal strength, interference or noise level, signal to noise ratio, or other signal quality criteria. In other words, the receiver system generally selects the best signal available for processing.
Frequency diversity systems also attempt to receive and process the highest quality signal. However, these systems generally include multiple radio receivers; one radio receiver functions as the operating receiver, and another receiver functions as a search and check receiver. The operating receiver stays tuned to the frequency of interest to receive and process the received radio signal. On the other hand, the search and check receiver searches for alternate reception frequencies that offer higher signal quality.
If the check receiver finds an alternate reception frequency, the operating receiver tunes to the alternate reception frequency and begins processing the received signal. Alternatively, the check receiver and the operating receiver may switch roles. In other words, the check receiver remains tuned to the alternate reception frequency and assumes the role of the operating receiver. The prior operating receiver then assumes the role of the check receiver and begins searching for alternative reception frequencies offering higher signal quality. In automobile radios, the operating receiver is sometimes referred to as the “audio receiver,” while the check receiver is sometimes referred to as the “background receiver.”
In the past, a change to a new radio signal source often introduced noticeable artifacts into the output signal produced by the radio system. One source of artifacts was the significant variation in propagation delay experienced by radio signals in reaching the radio system, either at different antennas, or at a different frequency. Due to the variation in propagation delay, there was no expectation that the new signal source would be aligned in time with the previously received and processed signal source. Thus, the switch to a new antenna or a new frequency resulted in artifacts caused by repetition or omission of information. When the output signal was a music program, for example, the artifacts detracted from the perceived quality of the program. Therefore, there is a need for additional improvements in the systems used for multipath reception that do not suffer from the shortcomings set forth above.